I would like to see real stats on this. Most people don't properly quantify availability and assume just because a big system goes down a lot that one they run could be more reliable, but it's usually apples and oranges.
- A large number of users will increase the likelihood that one of them will encounter a temporary issue; even if problems are intermittent, a single person using the system simply won't notice them, but lots of people will.
- More users tends to mean using more features, more system resources, etc, which increases the likelihood of an issue from either buggy/complex features, or resource exhaustion.
- Updates for a team typically involve being more careful and performing updates on a regular schedule, in order to minimize downtime.
- If a problem does occur and you're not around, the team is stuck until you are available. If you go on vacation or are hit by a bus, they're stuck longer.
- Having more users tends to require things like disaster recovery contingencies, security, etc. Somebody has to do this extra work, which is a cost in time and labor.
Where self hosted shines is in keeping complexity down and changes minimal. GitHub is a giant system, making it more likely for problems to interrupt service. They constantly ship changes, increasing the likelihood of interrupted service. A self hosted option can use the same simple system and version for much longer, only taking security patches until the version is EOL and needs to be upgraded.
But yeah, if you have no plan or organization, too many cooks is detrimental.